1. Field of the Invention
The present invention relates to a vibration actuator having a vibration element to generate vibration, and to generate relative motion between the vibration element and a relative motion member in compressive contact with the vibration element. More particularly, the present invention relates to an ultrasonic actuator having restriction members to regulate pitching vibration, and a method of producing the vibration actuator.
2. Description of the Related Art
Vibration actuators which generate vibrations in the ultrasonic regions are known as ultrasonic actuators or ultrasonic motors. A conventional vibration actuator using two simultaneously generated degenerate modes of vibration having different form is disclosed, for example, in "Fifth Collected Papers, Fifth Dynamics Symposium Related to Electromagnetic Force", page 393, Tomikawa (hereinafter "Tomikawa").
FIG. 22 is a perspective diagram of a vibration actuator 1 having a vibration element 2 disclosed by Tomikawa. Moreover, FIG. 23 is diagram illustrating a side view of the vibration element 2 and an example of a waveform of two (2) vibrations L1, B4 generated in the vibration element 2. As shown in FIG. 22, the vibration element 2 includes an elastic member 3 and an electromechanical energy converter (referred to hereinbelow as a "piezoelectric member") which converts electrical energy into mechanical energy. The elastic member 3 has a rectangular plate form and is a metallic material having a large resonant sharpness. The piezoelectric member 4 is mounted on one flat side of the elastic member 3. Moreover, two drive force output members 3a, 3b are formed protruding on a flat side of the elastic member 3 opposite the side on which the piezoelectric member 4 is mounted.
As shown in FIG. 22, the piezoelectric member 4 includes four (4) connected regions: input regions 4a, 4b to which two (2) phases A and B of drive voltage V.sub.A, V.sub.B are respectively applied; a detection region 4p which monitors the vibration state of the vibration element 2, and a ground region 4g. Silver electrodes 5a, 5b, 5p and 5g, for example, are mutually separately mounted on respective regions 4a, 4b, 4p, 4g.
Sliding members, (not shown in the drawing) formed of a high molecular material as a main component are affixed to the bottom surface of the drive force output members 3a, 3b. A relative motion member 6 is caused to be in compressive contact with the elastic member 3 via the sliding members by a suitable compressive force.
Moreover, the dimensions of the elastic member 3 are set such that the frequencies of the first order longitudinal vibration L1 and the fourth order bending vibration B4 about coincide. Furthermore, the drive force output members 3a, 3b are arranged in the length direction of the vibration element 2 in positions coinciding with the outside antinode positions l1, l4, among four (4) antinode positions l1, l2, l3 and l4 of the bending vibration B4.
As shown in FIG. 23, when high frequency drive voltages V.sub.A, V.sub.B with a .pi./2 phase displacement are applied to the elastic member 3, a first order longitudinal vibration L1, which vibrates in the length direction of the vibration element 2, and a fourth order bending vibration B4, which vibrates in the thickness direction of the vibration element 2, are simultaneously generated. The longitudinal vibration L1 and the bending vibration B4 generated in the elastic member 3 are combined, and the respective bottom surfaces of the drive force output members 3a, 3b are periodically displaced in elliptical form to generate an elliptical motion. As described above, the vibration element 2 generates relative motion between the drive force output members 3a, 3b and the relative motion member 6.
In the above-described manner, in a vibration actuator including the vibration element 2 having different modes of degenerate form, the longitudinal vibration L1 and bending vibration B4 generated in the elastic member 3 combine to generate elliptical motion in the drive force output members 3a, 3b, and to generate relative motion between the drive force output members 3a, 3b and the relative motion member 6. Accordingly, in the conventional actuator 1, it is necessary for the vibration element 2 and the relative motion member 6 to be placed in compressive contact by a suitable compressive force.
To apply a suitable compressive force, the present Applicant has proposed a compression member which compresses the vibration element 2 toward the relative motion member 6 at one position in the center portion with respect to the length direction of the vibration element 2 (the compression position C shown in FIGS. 22 and 23), as disclosed, for example, in Japanese Laid-Open Patent Publication JP-A-H8-140374.
Using the compression member disclosed in JP-A-H8-140374, the vibration element 2 can be reliably compressed toward the relative motion member 6 with a suitable compressive force, and with a very simple structure. Further, the compression member enables the elliptical motion generated in the vibration element 2 to be efficiently propagated to the relative motion member 6.
Moreover, as shown in FIG. 23, because the compression position C corresponds to the respective nodal positions of the longitudinal vibration L1 and the bending vibration B4 which arise in the elastic member 3, the vibrational attenuation accompanying the compression can be suppressed as much as possible. Because of this, the compression position C shown in FIGS. 22 and 23 was previously considered to be the most preferable position in order to design a vibration actuator which controls the vibrational attenuation accompanying compression, and performs reliable compression. Furthermore, heretofore, it was considered that reliable driving of the vibration actuator 1 was possible by compressing the vibration element 2 toward the relative motion member 6 at the compression position C shown in FIG. 23.
However, upon investigation by the present inventors, it was ascertained that by performing compression at the compression position C accompanying the driving of the vibration element 2, the two end portions in the length direction of the vibration element 2 vibrate, rising and falling in mutually opposite directions, centered on the compression position C. More specifically, the present inventors discovered that a pitching vibration may arise in the vibration element 2. One example of a direction of the pitching vibration is shown by the arrows in FIG. 23.
When the pitching vibration arises in the vibration element 2, noise having the frequency of the pitching vibration is generated, and the silentness, which is a characteristic feature of the vibration actuator, is lost. Moreover, a function (referred to hereinbelow as a "clutch function"), which is continuously propagated to the relative motion member 6 by the longitudinal vibration L1 and bending vibration B4 which arise in the vibration element 2, becomes insufficient, and driving efficiency falls.
Moreover, because the conventional vibration actuator does not include means to control the pitching vibration, the pitching vibration continues to be generated.